In contrast with small-molecule therapeutics and probes, which often readily penetrate biological membranes, larger macromolecules, such as peptides and proteins, are generally excluded from the cell interior. Given the vast array of applications for protein-based tools and therapeutics inside cells, there is great interest in developing safe and efficient protein delivery platforms that direct biologics into cells. To date, numerous approaches have been investigated to facilitate protein entry into the cytoplasm of cells, including cell-penetrating peptides, lipid-based molecules, nanoparticles, encapsulated protein containers, zinc-finger proteins, and super-charged green fluorescent proteins. Though each is capable of delivering protein cargo into cells to varying degrees, general mechanism-based limitations exist for these platforms. Cell-selectivity and/or efficiency-of-delivery remain particularly elusive features for most platforms owing to their shared nonspecific mode of interaction with membranes.
Platforms enabling targeted delivery of proteins into cells are needed to fully realize the potential of protein-based therapeutics with intracellular sites-of-action. As such, there remains a pressing need for delivery platforms with robust capacity that offer the possibility to deliver diverse protein-based therapeutics into specific cells.